The present invention relates to disk brakes and more particularly to improvements in a disk brake shoe used in larger area contact disk brakes for vehicles.
The disk brake shoe of the present invention is used in disk brake assemblies of the type described in the Applicant""s PCT application of PCT/CA97/01014, entitled IMPROVED DISK BRAKE ASSEMBLY and published as W098/29671 on Jul. 9, 1998. The disk brake assembly refers to full annular disk brakes proposed for automobiles and light trucks.
The full annular disk brake assembly described in this PCT application generally includes a housing mounted to a vehicle and a rotor disk mounted to a wheel of the vehicle. First and second annular brake shoes extend parallel to the rotor disk within the housing and are mounted thereto. The second brake shoe is movable axially by means of an oil applied bladder mounted to the housing. The oil applied bladder moves the second brake shoe axially against the rotor disk. When pressure is applied to the rotor disk by the second brake shoe and the bladder, the rotor disk is adapted to slide axially towards the first brake shoe so that the rotor disk frictionally engages the first and the second brake shoes.
It has been found that vibrations between the first and second brake shoes are the major cause of brake squeal. Under certain situations, vibrations may cause large displacements and severe stresses in the brake. The vibrations generally are produced by the frictional forces between the brake shoes and the rotor disk, especially when the frictional surfaces of the rotor disk are imperfectly machined or unevenly worn.
In order to reduce the vibration in the disk brake assembly elastic rolling seals are provided between the axially slidable rotor disk and the hub adapter, and between the second slidable brake shoe and its support structure. For further improving vibrations, an improved housing is described in the Applicant""s co-pending Canadian patent application 2,314,547 which is entitled DISK BRAKE HOUSING and filed on Jul. 25,2000. The housing includes an annular radial wall having an annular ridge on the inner surface to abut the brake shoe which is supported by the housing so that during a brake action uneven and variable forces applied to the brake shoe will cause local portions of the brake shoe to oscillate about the contact point. The oscillation will be dampened by damping pads placed between the housing and the brake shoe. The improvement made in the above two patent applications greatly reduce the vibration caused by uneven and variable forces during brake actions. However, there is still a need for additional improvements to further reduce the vibrations.
Imperfect machining or uneven wearing of the frictional surfaces of the rotor disk not only produce vibrations but also cause other problems. Protruding spots on the frictional surfaces of the rotor disk, resulting from either imperfect machining or uneven wearing, will be pressed to the brake shoes with much greater pressure when the rotor disk is frictionally engaged with the brake shoes during the brake action, which results in local high temperatures at those protruding spots to make those protruding spots crystallize. The crystallized protruding spots are very hard and make the brake shoes wear out rapidly.
Furthermore, the frictional movement between the rotor disk and the brake shoes produces a great amount of heat which causes thermal expansion of the brake shoes. The second brake shoe has an annular piston member protruding axially from the brake shoe, to be pressed by the flexible annular bladder so that when pressurized fluid is pumped into the bladder, the bladder is expanded and presses the brake shoe to move axially towards the rotor disk. The annular piston member has a diameter substantially equal to the diameter of the annular bladder. However, because of the thermal expansion of the brake shoe, the diameter of the annular piston member will increase and the annular piston member is not adapted to maintain its original radial position with respect the annular bladder. The radially displaced annular piston member forces a radial displacement of the flexible annular bladder when the annular piston member is compressed by the annular bladder, resulting in additional stresses in the flexible material of the bladder, and eventually causing premature aging of the flexible material, and finally inducing misfunction of the bladder. Therefore there is a need for further improvements to overcome the above problems.
It is an object of the present invention to provide a disk brake shoe, especially for a full annular disk brake for automobiles, that has an improved configuration to add a better reduction of vibrations.
It is another object of the present invention to provide a disk brake shoe having a longer lifetime.
It is yet another object of the present invention to provide a disk brake shoe adapted to keep a radial contact position with the flexible annular bladder regardless of the thermal expansion of the brake shoe.
It is a further object of the present invention to provide an improved disk brake assembly overcoming the shortcomings of the prior art disk brake assemblies.
Generally a disk brake assembly for a vehicle wheel on a vehicle includes a housing having an annular radial wall mounted to the vehicle, an annular rotor disk within the housing and co-axially mounted to the wheel, a brake shoe, and an annular expandable bladder attached to the annular radial wall of the housing, upon expansion of the bladder during a brake action, the brake shoe being adapted to move towards the rotor disk resulting in frictional engagement between the rotor disk and the brake shoe.
In accordance with one aspect of the present invention, the brake shoe comprises a radial section parallel to the rotor disk; means associated with the housing for radially supporting the brake shoe within the housing; means associated with the housing for preventing the brake shoe from rotation. The radial section is made from a resilient metal material and has a relative thin thickness whereby during the brake action the brake shoe is deformable under uneven and variable forces resulting from imperfect machining or uneven wearing of the rotor disk. The radial section of the brake shoe is deformed locally and instantly when it comes into contact with the protruding spots on a planar frictional surface of the rotor disk so that the high pressure between the rotor disk and the brake shoe applied by those protruding spots are minimized and the crystallization of those protruding spots is avoided. As a result, the rapid wearing-out of the brake shoe or brake pad attached to the brake shoe is prevented.
The brake shoe preferably comprises an annular brake pad for frictional engagement with the rotor disk, and a layer of resilient and deformable material disposed between the annular brake pad and a radial annular surface of the annular radial section of the brake shoe to secure the annular brake pad to the radial section. The layer of resilient and deformable material is also adapted to be instantly and locally deformed in a manner similar to the radial section of the brake shoe when the brake pad comes into contact with the protruding spots on the planar frictional surface of the rotor disk, to reduce the high pressure applied by those protruding spots during brake actions. The resilient properties provided by the layer of resilient and deformable material and the deformable radial section of the brake shoe not only reduce the high pressure between the rotor disk and the brake shoe to prevent the rapid wearing-out of the brake pad, but also help in absorption of vibration energies produced during brake actions to further reduce vibrations. Alternatively, the annular brake pad might be replaced by a plurality of brake pads circumferentially spaced apart for frictional engagement with the rotor disk. The layer of resilient and deformable material is disposed between the respective brake pads and the radial annular surface of the annular radial section of the brake shoe to secure the respective brake pads to the radial section.
In accordance with another aspect of the present invention, the brake shoe comprises a radial section having a radial planar surface parallel to the rotor disk; means associated with the housing for radially supporting the brake shoe within the housing; means associated with the housing for preventing the brake shoe from rotation; an annular piston member protruding axially from the annular radial section for being pressed by the annular expandable bladder; and a layer of resilient and deformable material disposed between the annular piston member and the radial section to secure the annular piston member to the radial section, permitting a relative radial displacement therebetween whereby the annular piston member is adapted to maintain an original radial position with respect to the annular expandable bladder regardless of radial thermal expansion of the annular rotor disk. The layer of resilient and deformable material disposed between the radial section of the brake shoe and the annular piston member is radially deformed when the brake shoe is thermally and radially expanded during brake actions while the annular piston member is held in the original radial position with respect to the annular flexible bladder, and as a result, radial displacement of the flexible bladder is avoided. The radial displacement of the flexible bladder, if it occurs might induce stresses and premature aging of the bladder, and eventually cause misfunction.
It is also important to prevent the heat energy produced from frictional movements during brake actions from transferring to the annular piston member because the annular piston member will thermally and radially expand itself when it is heated. Therefore, a thermal insulator is preferably disposed between the layer of resilient and deformable material and the annular piston member to inhibit heat transferring from the radial section of the brake shoe to the piston member. It is desirable that the piston member has a hollow structure defining an inner annular space within the annular piston member and a plurality of bores preferably extending therethrough to communicate with the inner annular space and the outside of the annular piston member so that air is enabled to enter and exit from the annular piston member to further ensure that the piston member is not improperly heated.
The improvements to the brake shoe according to the present invention are relatively simple but efficiently improve the performance of the disk brake assembly, preventing the premature wearing-out and misfunction of the disk brake assembly. Other advantages and features of the invention will be better understood with reference to a preferred embodiment of the invention described below.